Keats uses the ancient tradition of an ode, or ballad, as his poetic device. An ode is not unlike lyrics, as the words are meant to be put to music. In older times, odes had a very rigid pattern, but by Keats' day, it was the feeling rather than adherence to method that was important. It is that feeling in a familiar form that the poet uses to communicate.
While Keats was not as strict as the ancients, he did utilize form; the poem is not free verse. Each stanza consist of five sets of ten line stanzas; each stanza has its own rhyme scheme:
Thou still unravish'd bride of quietness,
Thou foster-child of silence and slow time,
Sylvan historian, who canst thus express
A flowery tale more sweetly than our rhyme:
What leaf-fring'd legend haunts about thy shape
Of deities or mortals, or of both,
In Tempe or the dales of Arcady?
What men or gods are these? What maidens
What mad pursuit? What struggle to escape?
What pipes and timbrels? What wild ecstasy?
Additionally, each stanza presents a problem in the first four lines (the quatrain). The last six lines, or sestet, gives an answer.
The style section of eNotes explains the first stanza this way: While the quatrain tells us that the poet cannot adequately express the "flowery tale" depicted on the urn, the sestet reveals why. The urn's pictures raise a string of questions that language alone cannot answer.